1. Field of the Invention
The present invention relates to the technical field of detecting an external force such as acceleration, pressure, flow rate of a fluid, magnetic force, or electrostatic force by using a piezoelectric plate such as a crystal plate to detect the size of the external force acting on the piezoelectric plate based on an oscillating frequency.
2. Description of Related Art
Some examples of an external force that acts on a system are a force that acts on an object based on acceleration, pressure, flow rate, magnetic force, electrostatic force, and the like, and there are many cases in which such external forces must be measured accurately. For example, during the course of development of an automobile, the force of impact on the seats when the automobile collides with an object is measured. Further, the acceleration of shaking during an earthquake must be investigated with as much precision as possible in order to examine the vibrational energy and amplitude of the earthquake.
In addition, as other examples of measuring external forces, there are cases in which the flow rate of a fluid or gas is accurately investigated and the detected values are reflected in a control system, or cases in which the performance of a magnet is measured.
When taking such measurements, it is desirable for the measuring apparatus to measure with high accuracy using a structure that is as simple as possible.
Patent Document 1 discloses that a piezoelectric film is cantilevered and deforms due to a change in a surrounding magnetic force, and thereby a change occurs in a current flowing to the piezoelectric film.
Patent Document 2 discloses providing a capacitively-coupled pressure sensor and a crystal unit that is arranged in a space that is separated from a region in which the pressure sensor is arranged, and connecting a variable capacitance of the pressure sensor and the crystal unit in parallel. Patent Document 2 further discloses that the pressure is detected according to a change in an anti-resonance point of the crystal unit upon a change in the capacitance of the pressure sensor.
The principles disclosed Patent Documents 1 and 2 are completely different than those of the present invention.    [Patent Document 1] Japanese Patent Application Laid-Open (JP-A) No. 2006-138852 (Paragraphs [0021] and [0028])    [Patent Document 2] Japanese Patent Application Laid-Open (JP-A) No. 2008-39626 (FIGS. 1 and 3)